1. Field of the Invention
The present invention relates to a vehicle speed detecting apparatus for detecting a traveling speed of a vehicle, particularly to a vehicle speed detecting apparatus for taking an input vehicle-speed signal whose period changes with the vehicle speed and producing an output a vehicle-speed pulse signal having a desired period by altering the period of the input vehicle-speed signal by any desired ratio.
2. Related Art
An existing vehicle speed detecting apparatus (vehicle speed sensor) for a speedometer obtains a vehicle-speed pulse signal from a first gear for driving the speedometer and a second gear for rotating a magnet in a transmission of a vehicle. The vehicle speed determined by detecting the rotational speed of the magnet. In this case, a vehicle-speed pulse signal having a standardized period is obtained by setting the gear ratio between the first and second gears correspondingly for each type of vehicle or the like and thereby adjusting the rotational speed of the magnet.
However, a gear-approach-type vehicle speed sensor for directly detecting the rotational speed of a gear has recently been developed as disclosed in U.S. Pat. No. 4,922,197 or U.S. application Ser. No. 08/162,625. For example, there is a gear-approach-type vehicle speed sensor which employs an MRE (Magnetic Resistance Element) to directly detect the rotational speed of the gear. In the case of the gear-approach-type vehicle speed sensor, however, it is impossible to adjust a gear ratio each type of vehicle or the like because neither the first gear for driving the speedometer nor the second gear for rotating the magnet are employed.
Therefore, it is necessary to change the number of teeth of the gear to detect the rotational speed even for the same transmission correspondingly to each type of vehicle or the like. However, when a frequency dividing ratio expressed by a fraction is necessary, it is impossible to obtain the desired frequency dividing ratio by changing the number of teeth of the gear because the number of teeth of a gear is a natural number. Therefore, a gear-approach-type vehicle speed sensor requires a circuit for changing the frequency dividing ratios.
Various dividing circuits by which any frequency dividing ratio can be set have been proposed. For example, a vehicle speed detecting apparatus by which any frequency dividing ratio can be set is disclosed in of Japanese Patent Laid-Open No. Hei. 4-198756.
In this vehicle speed detecting apparatus, however, dividing is performed merely at a frequency dividing ratio of 1/n and therefore, it is impossible to divide at an optional frequency dividing ratio M/D (M and D are natural numbers).
Japanese Patent Laid-Open No. Sho. 64-68616 discloses an apparatus using a vehicle distance recorder in which dividing is performed at a frequency dividing ratio M/D.
In this apparatus, however, the frequency dividing ratio is obtained by changing frequency dividing ratios at a certain period. Therefore, when changing the frequency dividing ratios before one period of an output pulse signal terminates, the period of an output pulse signal fluctuates when the dividing ratios are changed. Thus, the apparatus can be applied to a vehicle distance recorder but it cannot be applied to a meter such as a speedometer which quickly responds to the output pulse signal because a pointer thereof swings.